The present invention relates to an improved die for manufacturing tubes of the type commonly known as `J-tubes` i.e. tubes consisting of a straight-sided portion joined to one limb of a curved generally U-shaped portion, the straight and curved portions lying in the same plane. Up to a few years ago, J-tubes were made of lead or copper, but now most are made of a rigid plastics material such as polypropylene, since plumbing regulations often do not permit the use of J-tubes made of flexible materials. As used herein, the term `rigid` material means a material which cannot be substantially distorted in shape without damage, although it is sufficiently flexible to allow slight (e.g. a few millimetres) distortion without damage. However, such material is normally flexible when heated, as during a moulding process, for example a thermoplastic.
Most plastics J-tubes are made by injection moulding the plastics material into a suitably shaped die, the lumen of the tube being occupied by a core positioned in the die. Clearly, it is advantageous to make the J-tube in one piece if possible, but since the J-tube must be made of a rigid material, it is difficult to extract the cor from the curved portion of the `J` tube because the core emerging from the curved portion strikes the straight portion of the tube.
This problem has been overcome in a number of ways:--by using a multi-part core for the curved portion, or by using conventional die and a two-part core, one part for the straight portion of the tube and one part for the curved portion, and removing the curved part of the core by distorting the tube to force the curved part of the core past the straight portion of the tube. Considerable force is needed to extract this curved core part and the tube and cores together must be removed from the die before the curved core part is extracted. The extraction is done manually or part-manually, which of course slows down the whole forming operation. The use of a multi-part core for the curved, portion of the tube reduces the distortion needed to extract the core, but again slows the forming operation because the core must be re-assembled after each tube is formed.